Acetylcholine: a neurotransmitter for learning and memory?
The cholinergic hypothesis claims that the decline in cognitive functions in dementia is predominantly related to a decrease in cholinergic neurotransmission. This hypothesis has led to great interest in the putative involvement of the cholinergic neurotransmission in learning and memory processes. This review aims to assess the data of studies in which the role of acetylcholine (ACh) in cognitive functions was investigated. For this purpose, studies from three different fields of research, namely: (1) behavioral pharmacology (effects of drugs on behavior); (2) behavioral neuroscience (effects of brain lesions on behavior); and (3) dementia, are discussed separately. The experimental tools that have been used in pharmacological studies may appear to be inadequate to enable conclusions to be drawn about the involvement of ACh in learning and memory processes. Especially, the use of scopolamine as a pharmacological tool is criticized. In the field of behavioral neuroscience a highly specific cholinergic toxin has been developed. It appears that the greater and more specific the cholinergic damage, the fewer effects can be observed at the behavioral level. The correlation between the decrease in cholinergic markers and the cognitive decline in dementia may not be as clearcut as has been assumed. The involvement of other neurotransmitter systems in cognitive functions is briefly discussed. Taking into account the results of the different fields of research, the notion that ACh plays a pivotal role in learning and memory processes seems to be overstated. Even when the role of other neurotransmitter systems in learning and memory is taken into consideration, it is unlikely that ACh has a specific role in these processes. On basis of the available data, ACh seems to be more specifically involved in attentional processes than in learning and memory processes.